Recently, the number of users of mobile communication equipment is increasing rapidly, and hence there has been greater demand for more effective utilization of limited width frequency bands. For this reason, a band-pass filter (in particular, a filter utilized on the side of a base station under a microwave band environment) is required to have a steep cutoff characteristic and a low power loss performance in the pass-band. To implement a filter having a steep cutoff characteristic under a microwave band environment, the number of filter stages shall be increased. However, if the filter is composed of an ordinary conductive metal, the power loss in the pass band becomes excessively large.
If the filter employs a superconductive material which has a low surface resistance in the microwave band, the filter will have very little loss in the pass-band. Particularly, there are many reports available in which it is stated that a so-called “superconductive microstrip filter” has achieved a filter which makes it easy to design the arrangement thereof and attain miniaturization of the same.
FIG. 15 is a plan view schematically showing a superconductive microstrip filter. As shown in FIG. 15, a superconductive microstrip filter 50 has a dielectric substrate 53 (made of MgO or the like) having a desired line pattern of a superconductive film (superconductive signal line portion) 5la, 51b and 52 formed by means of lithography or the like, an input connector 54a to which a signal input coaxial cable can be connected, and an output connector 54b to which a signal output coaxial cable can be connected. FIG. 16 is a cross sectional view taken along the line A—A on the superconductive film 52 (51a and 51b) shown in FIG. 15.
The above-described input connector 54a is bonded together with the superconductive film 51a at a center conductor 55 thereof by using a solder or the like so that when the input connector 54a is connected with the coaxial cable 65a (See FIG. 15), an input microwave can be transmitted through the coaxial cable 65a and led into the superconductive film 51a. Similarly, the output connector 54b is bonded together with the superconductive film 51b at a 55 center conductor 55 thereof by using a solder or the like so that a microwave outputted through the superconductive film 51b can be inputted into the coaxial cable 65b (See FIG. 15). In FIG. 15 reference numerals 55a and 55b designate these bonding portions.
Each of the superconductive films 52 (See FIG. 15) is optimally designed in its length and the distance from it to the neighboring superconductive film 52 (forming a coupling capacity together with that superconductive film) so that the superconductive film serves as a resonator which resonates a particular frequency (or wavelength) component in the frequency band of the input microwave components introduced into the above-described superconductive film 51a. In this way, only the particular frequency (or wavelength) component in the frequency band of the input microwave components introduced into the above-described superconductive film 51a is resonated in each of the superconductive films 52 and propagated to the adjacent superconductive film 52. Finally, the particular frequency component in the frequency band is extracted from the superconductive film Sib and outputted through the output connector 54b to the coaxial cable 65b. 
In the above example, the number of pieces of the superconductive film 52 (in the example shown in FIG. 15, the number is five) corresponds to the filter stage number which decides the cutoff characteristic of the filter assembly. As the number of filter stages is increased, the cutoff characteristic becomes steeper. The above superconductive films 51a, 51b, 52 are formed of a superconductive material (chemical compound) composed of YBCO (i.e., Y—Ba—Cu—O: in this case, symbol Y represents yttrium, Ba barium, Cu copper, and O oxygen, respectively).
When the above-described superconductive micro-strip filter 50 (hereinafter sometimes simply denoted as “superconductive filter 50”) is operated, the filter is housed within a package 61 made of an ordinary conductivity metal having a high thermal conductivity and a low thermal expansion (shrinkage) ratio such as copper, INVER or the like, as schematically shown in FIG. 17. Then, the package 61 is disposed on a cold head (cooling medium) 63 provided in a vacuum heat insulating vessel 62 (reference numeral 64 represents a vacuum space). The cold head 63 is connected to a refrigerator not shown and the superconductive films 51a, 51b and 52 are cooled (to about 70K (Kelvin)) by the refrigerator, whereby the superconductive films are placed in a superconductive state.
The structure 67 shown in FIG. 17 is hereinafter referred to as “superconductive filter module 67”. FIG. 17 schematically shows the superconductive filter module 67 in which only the vacuum heat insulating vessel 62 is shown as a cross-sectional side view (that is, FIG. 17 includes the superconductive filter 51 as viewed from the arrow B in FIG. 15). Further, in FIG. 17, reference numerals 65c and 65d represent coaxial cables similarly arranged to the coaxial cables 65a and 65b, and these coaxial cables are connected to the coaxial cables 65a and 65b through the connectors 62a and 62b provided on the vacuum heat insulating vessel 62, respectively.
Meanwhile, as an index indicative of the performance of the refrigerator, there is a refrigerator output. This index corresponds to a heat amount flowing into the vessel as a heat load allowable for the refrigerator to keep the cooling object at a low constant temperature. If the requested cooling condition is a cooled state at a temperature of 70K, the value of the index is set to about several W (watt) in terms of reasonable balance with the power consumption of the refrigerator.
It is true that, in the above-described conventional superconductive filter module 67, it is attempted to keep the package 61 at a constant low temperature (about 70K) within the vacuum heat insulating vessel 62 with the refrigerator. However, as described above, the center conductors 55 of the input connector 54a and the output connector 54b are bonded together with the superconductive films 51a and 51b by means of solder or the like (bonding connection). Thus, heat flows from the coaxial cables 65c and 65d which are exposed under the external temperature (room temperature) outside the vacuum heat insulating vessel 62 through the coaxial cables 62a and 62b (external conductors mainly constituting the coaxial cables 62a and 62b) into the package, leading to temperature increase at the bonding portions 55a and 55b, with the result that the surface resistance of the superconductive films 51a and 51b is increased at the bonding portion. As a result, the whole loss of the superconductive filter 50 is increased.
Further, the bonding materials utilized at the bonding portions 55a and 55b differ from each other in thermal expansion coefficient. Thus, the bonding portions 55a and 55b will suffer from damage, for example, under low temperature conditions such as of 70K, and contact at the bonding portion becomes unsatisfactory, with the result that the bonding state becomes unstable. This means that a desired filtering characteristic cannot be obtained.
Furthermore, according to the above arrangement, metal surfaces (conductive materials) contact each other throughout the external conductors of the coaxial cables 65a and 65b, the input connector 54a, the output connector 54b, the package 61, and the cold head 63. Therefore, heat can be conducted from the outside through the metal surface connection and finally allowed to flow into the refrigerator, thereby increasing the load imposed on the refrigerator.
Although the amount of heat flowing into the package per coaxial cable depends on the material thereof, the dimension thereof or the like, it can be estimated to be about 1 W. However, a single refrigerator unit can be connected with several cables such as cables for input and output, cables for transmission and reception, and so on. In some cases, the single refrigerator unit can be connected with several tens of cables for each communication channel or sector, depending on the arrangement of the communication system.
In this case, the total amount of heat conducted from the outside to the refrigerator will far exceed the permissible amount of heat [several W (watt)] flowing into the refrigerator, with the result that the superconductive filter 50 cannot be maintained in the superconductive state satisfactorily (i.e., the loss becomes large).
Furthermore, when an electric current is allowed to flow in the superconductive film 52 (51a, 51b) of the single unit of the superconductive filter 50, the electric current density profile becomes one in which the current flows intensively at the edge 52a thereof as shown with an imaginary line in FIG. 16 (i.e., the current density becomes high at the edge 52a). This phenomenon is referred to as “edge effect”). For this reason, not only the Q-value (index of sharpness of passing characteristic) of the superconductive filter 50 but also the power withstand performance of the superconductive filter 50 are limited. For example, the above-described superconductive filter 50 has a power withstand performance of about several watts. Thus, this filter is applicable to receiving side of radio communication equipment (e.g., a base station) but not applicable to the transmission side of the same in which power withstand performance of several tens to several hundreds or more is required.
The present invention was made in view of the above. Therefore, it is an object of the invention to provide a superconductive filter module and a superconductive filter assembly in which heat conduction from the outside can be suppressed as far as possible, the superconductive condition can be created with stability, with the result that a stable filtering characteristic can be created, and power withstand performance becomes excellent, and hence even if the number of stages of filters is increased to attain a steep cutoff characteristic, the loss deriving from the increased number of stages can be suppressed to the minimum level.
Also, an object of the present invention is to provide a heat insulating type coaxial cable which can suppress heat flow into a superconductive device such as a superconductive filter assembly to the minimum level.